1. Field of the Invention
The present invention relates to an ink jet recording apparatus (hereinafter, may also be referred to as a liquid jet recording apparatus). More particularly, the invention relates to an ink jet recording apparatus that performs its recovery operation by circulating ink between the recording head and ink tank.
2. Related Background Art
As has been represented by ink jet recording apparatuses, it is necessary to make an arrangement so that recording liquid (hereinafter, may also be referred to as ink) can flow in a path whose sectional area is extremely fine, and that the apparatus should be provided with means for preventing such path from being clogged by dust in ink, educt of ink or bubbles in ink. For an ink jet recording apparatus, a dust removal device, which is provided with a filter for removing fine dust and air bubbles, is installed on the ink supply system that carries ink from an ink tank to discharge ports through a common liquid chamber so as not to cause the discharge ports that discharge ink droplets to be clogged by dust and other educt.
FIG. 4 and FIG. 5 are cross-sectional views which schematically illustrate the entire structure of such conventional supply recovery system and dust removal device, respectively.
The dust removal device shown in FIG. 4 and FIG. 5 is used for a structure that executes a recovery process by circulating ink.
When ink is carried from the ink tank to the recording head through the ink forwarding path or returning path at the time of recording, it is usual that the ink is not carried by the application of any pressure exerted by means of a pump, but by the application of capillary force or the like created by the recording head in cooperation of the supply of ink. At this juncture, air bubbles should pass the filter of the dust removal device. Such arrangement is necessary because if the air bubbles are caused to enter the recording head and reside on the ink path on the discharge port side, its discharge becomes unstable, and in the worst case, ink discharges are disabled.
In FIG. 5, the ink 2, which flows in from the ink tank in the direction indicated by an arrow a through the ink forwarding path or returning path at the time of recording, is carried through a flow-in tube 21 to the dust removal device 20 having a filter 1. The ink 2 that flows in the flow path 3 of the dust removal device 20 is filtered by means of the filter 1 that does not allow any dust particles or the like whose diameter is larger than 10 .mu.m to pass. By use of this filter 1, the dust particles 4 and air bubbles 5 in ink 2 are caused to stay on the side at a, that is, the upstream side of the filter 1. Then, dust particles and air bubbles do not flow to the side indicated by an arrow b, that is, the downstream side that corresponds to a flow-out tube 22.
On the other hand, at the time of recovery, ink is carried from the ink tank through the ink forwarding path to the ink head under pressure by means of a pump, and the ink is circulated further from the recording head under pressure to the ink tank through the ink returning path. By this circulation, the air bubbles residing in the ink forwarding path, returning paths, and recording head are caused to return to the ink tank, and then, released from the ink tank to the air outside.
At this juncture, the air bubbles should pass each filter of the dust removal device installed on the way of the ink forwarding path or ink returning path.
In order to allow the air bubbles to pass the filter, there is a need for the provision of a specific difference in pressure before and after the filter so as to break the meniscus formed by ink and a gaseous body on the surface of the filter.
In accordance with the prior art shown in FIG. 4, all the ink should pass the filter 1 of the dust removal device 6 when ink is carried under pressure by means of a pump at the time of a recovery operation, and then, a great difference is created in pressure before and after the filter due to the pressure loss at the filter 1; hence allowing the air bubbles to pass the filter.
The ink and air bubbles that have passed the filter 1 are caused to enter the common liquid chamber 14 from one end of the recording head 13. Thus, a part of ink flows from the discharge ports 15 to the outside, and expel the air bubbles in the nozzles at that time, while the remaining portion of the ink passes the common liquid chamber 14 and enters the dust removal device 7 installed on the ink returning path after flowing out from the other end of the recording head 13. As in the case of the dust removal device 6 installed on the ink forwarding path, air bubbles should pass the filter when a pressure difference is created before and after the filter 1 due to the pressure loss at the filter 1 of the dust removal device 7; hence being circulated to the ink tank.
However, because of the event that the pressure loss is created even at the dust removal device 7 in the ink returning path when being carried under pressure by means of a pump, the amount of ink flowing out from the discharge ports becomes considerably greater than the normal amount of ink to be required just for removing the air bubbles in the nozzles. As a result, the amount of waste ink becomes unfavorably greater in terms of running costs.
Also, the flow rate of ink passing the dust removal device 7 installed on the ink returning path becomes smaller than that of ink passing the dust removal device 6 installed on the forwarding path to the extent that ink should flow out from the discharge ports inevitably.
Therefore, in order to secure a specific difference in pressure (flow rate) before and after the filter on the returning ink path, too, it is necessary to make the capacity of such pump larger so that the ink flow rate is still sufficient on the dust removal device 7 on the ink returning path even after the ink is caused to flow out from the discharge ports. This arrangement unfavorably brings about the higher costs of the apparatus.
Now, the description will be made of problems that should be encountered when a recovery operation is performed by use of a pump whose flow rate is small.
In other words, whereas the air bubbles can still pass the filter of the dust removal device on the ink forwarding path by use of such a pump, the air bubbles cannot pass the filter of the dust removal device on the ink returning path. As a result, those bubbles residing on the common liquid chamber side of the filter are caused to enter the recording head at the time of recording operation, and then, disabled ink discharges may ensue.
The present inventors have measured the differences in pressure before and after the filters of the dust removal devices, and obtained the results shown in Table 1 given below. Numerical values in the Table 1 indicate the pressures at the entrance side of the dust removal device 6 (A); the exit side thereof (B); the entrance side of the dust removal device 7 (C); and the exit side thereof (D), respectively, provided that the flow rate of the pump is set at approximately 1.5 ml per second, and the area of the filter, approximately 30 mm.sup.2.
TABLE 1 ______________________________________ A B C D ______________________________________ Pressure (mmAq) 1550 980 310 200 ______________________________________
Observing the results shown in this table, it is understandable that the difference in pressure between C and D becomes smaller when the pressure drops down due to the flowing out of ink from nozzles of the conventional apparatus, and this event makes it impossible for the air bubbles to pass the filter.
In a case of an elongated head having many numbers of nozzles, the amount of ink that flows out is greater to the extent that the number of nozzles is increased. In this respect, the drawback described above is particularly conspicuous.